Activity-dependent synaptic plasticity is crucial for the development and function of the nervous system. From my previous studies, we have found that in simple networks of cultured neurons, (i) correlated pre- and postsynaptic spiking activity induces different types of synaptic modification depending on precise spike timing; and (ii) synaptic strengthening induced at one synapse appears to propagate to specific neighboring sites in a network. Such temporal and spatial specificity of spike timing-dependent plasticity (STDP) reflects intriguing cellular mechanisms and bears important consequences for the development and function of neuronal circuits. Here, we propose a series of experiments aimed at establishing a set of spatio-temporal rules for STDP and its propagation, and revealing the underlying cellular signaling mechanisms. In Aim 1, we will identify quantitative rules for the temporal integration of STDP, emphasizing the involvement of short-term plasticity and the interaction between potentiation and depression processes. In Aim 2, we will use a novel local activation method to characterize STDP at single or a small number of identified synaptic boutons and to characterize the directions, ranges and speeds of the propagation of STDP to other identified boutons. In Aim 3, we will combine imaging and electrophysiological techniques to investigate Ca 2+ dynamics in the induction and propagation of STDP, and to evaluate the contributions from different Ca 2+ sources. The proposed project, by pursuing a set of quantitative rules for synaptic modification and a mechanistic understanding of these rules, promises to bridge the gap between synaptic physiology and neural network behavior, and advance our understanding of how experience shapes the development and function of neuronal circuits in the brain. Ultimately, these studies will provide insights into and may lead to cures for human diseases in the nervous system, especially those related to learning and memory.